The trick here, I believe, is to realise that reduction of the [itex]33x[/itex] is probably not a feasible option at all - so what you want to get in the end is not a quadratic or polynomial equation, but a simple product of trigonometric functions being equal to a nice number.

So the hint is to use the trigonometric identity [itex]\sec^2 33x = 1 + \tan^2 33x[/itex] and keep the [itex]\cos 2x[/itex] term as it is.

Edit: I realised a much much simpler method. Note that the LHS is [itex]\geq 0[/itex]. How about the RHS? This gives you the answer rightaway.